High pressure low density polyethylene (HP-LDPE) is a complicated mixture of highly branched polymers with a very broad molecular weight and structural polydispersity that is nearly impossible to characterize. It was the first form of polyethylene (PE) to become commercially viable and was discovered in 1933. The basic process is the free radical polymerization of supercritical ethylene. Typically this means that the reactions are done at high pressure (over 150 MPa) and temperatures of 150 to 350° C. Some sort of free radical initiator is needed for these reactions, for example, oxygen or peroxides are commonly employed. Two main types of reactors are used—autoclaves and tubular. The polymers made in this process are characterized as having a highly branched structure. This is believed to come about from two main mechanisms.
In one mechanism, the free radical on the growing end of a chain can loop back (mainly five carbon loops) to some other portion of the chain. The loop breaks at the point of reaction, to which the radical is transferred. The loop then becomes a branch off of the chain, which continues to grow from the reaction point as depicted in prior art FIG. 1. This backbiting reaction mainly leads to short branches a few repeat units long (especially butyl branches). Much longer branches are produced from the second mechanism, in which the growing end of a chain terminates on another molecule as depicted in prior art FIG. 2. Several other mechanisms have also been proposed, and the complete suite of reactions that occur has not been firmly elucidated. It is clear though, that these mechanisms result in a very complicated, tree-like architecture with both short and long chain branches. These are called ‘low-density’ polyethylenes (LDPE) in contrast to the higher density, linear versions. The reason that the products of the high pressure process have lower density is that they are lower in crystallinity. Only long methylene sequences can participate in the paraffin-like crystals of PE, so the side branches serve to lower the crystallizability of LDPE. This effect is mainly due to the short branches (e.g., ethyl, butyl, hexyl) that arise from the backbiting mechanism, simply because there are many more of these. Since the frequency of such branching can be controlled by various process variables (Temperature, Pressure, initiators), so can the density or crystallinity of the polymers.
The lower density structures yield polymers that work well in many extrusion processes. The power needed to extrude the polymer through a die and blow film, for instance, in much less than that for a corresponding linear polymer, however there are poorer mechanical properties (for example tear strength and dart impact) compared to linear polymers. This is due to the long chain branching (LCB) of the HP-LDPE, and more precisely to the nature of the LCB in it, that is, the length, number, and placement of the branches in each molecule, and the distribution of these parameters among the chains. Moreover, the bubble that forms when the film is formed is more stable to rupture with HP-LDPE than LLDPE, allowing higher throughout rates. On the other hand, the mechanical properties of LLDPE films, such as tear strength and dart impact strength, are much greater than for HP-LDPE. To get a balance of both processability and film performance, many film manufacturers use blends of HP-LDPE and LLDPE, but these are clearly still far from the optimum that might be obtained.
U.S. Pat. No. 6,255,424 discloses a convergent method for making dendritic polymer structures via a single step (one pot), anionic polymerization process in a living polymer system. More particularly, a method is disclosed for making vinyl-containing dendritic polymer structures. The method yields a broad distribution of the number of branches and their corresponding length.
A need exists for an improved composition and synthesis method of producing highly branched saturated hydrocarbon polymers with improved control of the branch length, branch number and placement of the long chain branching (LCB parameters), such that the resulting polymer will have both superior processing and mechanical performance. More particularly, a need exists to prepare model comb polyethylenes with one, two, or more linear branches, using anionic polymerization of butadiene and controlled linking chemistry, followed by hydrogenation as disclosed in U.S. Pat. Nos. 6,355,757, 6,391,998, and 6,417,281, all of which are herein incorporated by reference. Still more particularly, a need exists for a synthesis method that yields well-defined branched saturated hydrocarbon polymer compositions, which have either branches on branches or tetrafunctional branched products.